Commercial, including sponsorship and merchandising, and broadcast revenues were the biggest contributors to clubs' earnings.

"Although only Manchester United and Real Madrid have gone past the 500 million euro revenue marker so far, we predict that all of the top five clubs may reach that point by next year," said Dan Jones, partner in the sports business division at Deloitte, in a report out Wednesday.

While Jones described Real Madrid as "the most successful European club side of all time," he said its commercial supremacy was under threat from Manchester United.

Jones' colleague, Austin Houlihan, said: "Despite a poor on-pitch season in 2013/14, United's commercial strategy of securing global and regional partners is delivering substantial growth. Commercial revenue has grown 83 percent in the last three years… If they can return to the Champions League in 2015/16 there is a strong possibility they could be top in two years' time."

Eight U.K. Premier League clubs ranked among Deloitte's top 20 earners, with Manchester City the best performer after United, making 414.4 million euros.

Strikingly, every club in the Premier League made it into Deloitte's top 40.

"The fact that all the clubs in the Premier League are in the top 40 is testament to the huge appeal of the league globally and also the equality of the distributions the clubs enjoy relative to their European counterparts," said Houlihan.